1. Field of the Invention
The present invention relates to a driving device for a recording head, an image recording apparatus, and a driving method for a recording head, and more particularly to circuit technology for reducing distortion in the output waveform caused by temperature change in the drive circuit unit.
2. Description of the Related Art
In general, an inkjet recording apparatus, which forms a desired image by ejecting and depositing ink droplets from a plurality of nozzles provided in an inkjet head onto a recording medium, is widely used as a generic image forming apparatus. A known ejection method for the inkjet head in the inkjet recording apparatus is one where pressure generating elements, such as piezoelectric elements or heat generating elements, are provided at a plurality of pressure chambers, which are connected respectively to the nozzles, and an ejection force is applied to the ink inside the pressure chambers by applying a prescribed drive voltage to the pressure generating elements so as to operate the pressure generating elements.
As a method for driving the piezoelectric elements, it is suitable to use a common waveform method in which drive waveform elements contained in a common drive voltage corresponding to different ink ejection volumes are applied selectively to the piezoelectric elements by using analogue switches, a multiplexer, or the like, thereby varying the ejection volume of the ink droplets ejected from the nozzles.
In general, a DC power supply device (DC-DC converter) is suitable for use in supplying a drive voltage to a thermal type of head, which uses the heat generating elements, or to a piezoelectric type of head, which uses the piezoelectric elements, and a flexible flat cable (FFC) is suitable for connection between the DC power supply device and the head. Since the wires provided in the FFC have wiring resistance, then voltage drop occurs in the transmitted drive voltage, and for example, in a head that uses the heat generating elements, variation arises in the amount of heat generated by the heat generating elements, leading to a variation in the ink ejection volume, and therefore non-uniformities arise in the ink density in accordance with this variation in the ink ejection volume, and the quality of the recorded image deteriorates. Moreover, since the wires that transmit the drive voltage also have a capacitive component and an inductive component, in addition to the resistance component, then these components give rise to waveform distortion in the drive voltage. Various methods have been proposed in order to eliminate waveform distortion of this kind which occurs in the drive voltage.
Japanese Patent Application Publication No. 2006-159573 discloses a recording apparatus including a serial type of recording head, in which a drive power supply circuit supplying a drive power to a recording head from a DC-DC converter is provided in the main body of the recording apparatus, thereby supplying power and control signals to the recording head. The recording apparatus has, on a carriage, a carriage circuit board having terminals for determining the output voltage from the DC-DC converter, and in the DC-DC converter, a capacitor is connected between a ground terminal for supplying the drive power and a ground terminal for determining the output voltage. The ground terminal for supplying the drive power and the ground terminal for determining the output voltage are connected on the carriage substrate, in such a manner that the voltage drop due to the wiring resistance of the power supply wires is cancelled out, and hence a stable power that is free of oscillations or fluctuations is supplied to the recording head.
However, in the common drive waveform method described above, variation occurs in the on-resistances of the internal drivers (e.g., circuit units including MOSFETs (metal-oxide-semiconductor field-effect transistors)) of the switch IC that selectively applies a part of the common drive waveform corresponding to the pressure generating elements, and this variation affects the drive voltage applied to the pressure generating elements.
FIG. 14 shows a driving device of the pressure generating elements (e.g., piezoelectric elements in FIG. 14) in the related art. The driving device 500 shown in FIG. 14 includes: a DC-DC converter 504, which supplies drive power to a liquid ejection head (hereinafter, called “head”) 502; a FFC 506, which connects the DC-DC converter 504 with the head 502; a shift register 512 and a latch circuit 514, which selectively apply the drive waveform generated by a common waveform generating unit (not shown) to the piezoelectric elements 510 on the basis of the image data; and a switch IC 518 including an output-stage push-pull circuit block 516. In FIG. 14, the resistance component and the inductive component contained in the wiring in the FFC 506 are denoted with reference numeral 520.
When a control signal corresponding to the image data is inputted to the switch IC 518, the switch IC 518 selects the drive waveform that is to be applied to each piezoelectric element 510 from the common drive waveform generated by the common waveform generating unit, and applies the drive voltage obtained by receiving the power supply from the DC-DC converter 504 and amplifying the power of the corresponding drive waveform, to the corresponding piezoelectric element 510.
A PWM (pulse width modulation) control method is used in the DC-DC converter 504 shown in FIG. 14. Although detailed description of the PWM control method is omitted here, in the DC-DC converter 504, the FET 511 is controlled by means of a pulse signal (modulated control signal) VP obtained by comparing the differential voltage (error voltage) ΔV between the output voltage VO and the reference voltage VREF, with a sawtooth waveform voltage Vth, and the output voltage is maintained at a uniform voltage by applying a pulse width modulation is applied to the input voltage VIN.
A smoothing circuit block constituted of a diode D, a coil L and a capacitor C provided on the downstream stage of the FET 511 supplies a voltage from the capacitor C while the FET 511 is on, and supplies a voltage from the diode D and the coil L while the FET 511 is off, thereby maintaining the output voltage VO at a uniform voltage. In other words, the DC-DC converter 504 generates a direct voltage (PWM waveform), and supplies the voltage to the switch IC 518 to drive the piezoelectric element 510.
In this case, since a peak current of the order of several amperes at maximum flows in the push-pull circuit block 516, which is located at the output stage of the switch IC 518, then the switch IC 518 generates heat as a result of this current. Hence, when the internal temperature is raised due to the heat generated by the switch IC 518, then the on-resistance (RON) of the drivers constituting the push-pull circuit block 516 becomes relatively large.
The temperature dependency of the on-resistance of a general MOSFET can be expressed as follows:R=R0×(T/T0)1.5,where R is the on-resistance at temperature T, and R0 is the on-resistance at temperature T0 (reference temperature).
When the on-resistance of the switch IC 518 has become relatively large in this way, waveform distortion occurs in the drive voltage 520 shown in FIG. 15A, and the drive waveform 530 shown in FIG. 15B is obtained. More specifically, waveform rounding corresponding to the time constant represented by the product of the increase in the on-resistance and the electrostatic capacitance of the piezoelectric element 510 occurs, as shown in the rising part 532 and the falling part 534 in FIG. 15B, and looking in particular at the rising part 532, for example, it can be seen that the rise time, which is originally 1 microsecond in FIG. 15A, increases to 1.4 microseconds in FIG. 15B. When waveform rounding of this kind occurs, then a problem arises in that the ink droplet ejection characteristics change (the ejection speed becomes slower).
In Japanese Patent Application Publication No. 2006-159573, no attention is paid to the on-resistance of the driving circuit units, and therefore waveform distortion occurs in the drive voltage supplied to the inkjet recording head, due to the above-described change in the on-resistance, and deterioration in the ejection characteristics due to the change in the on-resistance cannot be avoided.
Furthermore, the on-resistance of the driver of the switch IC displays temperature dependency that also displays dependency on location. For example, a line type head 550 having a nozzle row of a length corresponding to the full width of the recording medium, such as that shown in FIG. 16 tends to have a higher temperature in the central portion than in the end portions, and a switch IC (not shown) located in the central portion of the head 550 therefore has a higher on-resistance than a switch IC (not shown) located in the end portion of the head 550. Accordingly, the drive voltage that is applied to the piezoelectric elements (not shown) in the central port of the head 550 has greater waveform rounding than the drive voltage that is applied to the piezoelectric elements (not shown) in the end portions of the head 550.
Consequently, the ink droplets ejected from the nozzles (554 to 560 in FIG. 16) in the central portion of the head 550 suffer a decline in the ejection speed. If the ejection speed falls, then landing position displacement occurs as indicated, for example, by the dots 562 and 564 formed by the ink droplets ejected from the nozzle 554 and nozzle 560, and thus, image non-uniformities occur. As shown in FIG. 17, taking the boundaries to be ±1σ when the temperature distribution of the head 550 is taken to be a normal distribution, then the region inside these boundaries is defined as the central portion of the head and the regions outside these boundaries are defined as the end portions of the head.